Fix frivolous usage of precise timers in Krita (and fix VSync sychronization with Qt)

Extract from HACKING:

We should be carefult about allocation of timers in Krita. The number of precision timers is limited on some platforms (e.g. Windows). When Qt runs out of precision timers it silently degrades all newly created timers into non-precision ones. That can affect timers that are used for the canvas framerate stabilization and cause tearing and FPS drops.

Rules of thumb:

1. When using QTimer::singleShot(), either use null timeout to just push the event through the queue, or pass Qt::CoarseTimer as the second argument:

   QTimer::singleShot(0, ...);

   or

   QTimer::singleShot(, Qt::CoarseTimer, ...);

   If you really-really-really need a precise single-shot event, then pass Qt::PreciseTimer explicitly.

   The problem is that QTimer::singleShot() creates a precise timer by default, which can consume all the available precise timers in the system for the app.

2. When creating timers or using KisSignalCompressor, set timeout to values strictly higher than 20 ms (unless you really know what you are doing). Value 25 ms is a good default to compress updates for the lightweight widgets, and value 100 ms is a good default to compress updates for heavy widgets.

   The problem is that Qt internally converts all the timers into precise ones, if their timeout is less or equal 20. Which can also consume all the available precise timers.

CC:kimageshop@kde.org